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2011•BMJOpen access

How to obtain the P value from a confidence interval

Douglas G. Altman, Martin Bland

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Abstract

We have shown in a previous Statistics Note 1 how we can calculate a confidence interval (CI) from a P value. Some published articles report confidence intervals, but do not give corresponding P values. Here we show how a confidence interval can be used to calculate a P value, should this be required. This might also be useful when the P value is given only imprecisely (eg, as P<0.05). Wherever they can be calculated, we are advocates of confidence intervals as much more useful than P values, but we like to be helpful. The method is outlined in the box below in which we have distinguished two cases. #### Steps to obtain the P value from the CI for an estimate of effect ( Est ) ##### (a) P from CI for a difference

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What this paper is about

We have shown in a previous Statistics Note 1 how we can calculate a confidence interval (CI) from a P value. Some published articles report confidence intervals, but do not give corresponding P values. Here we show how a confidence interval can be used to calculate a P value, should this be required. This might also be useful when the P value is given only imprecisely (eg, as P<0.05). Wherever they can be calculated, we are advocates of confidence intervals as much more useful than P values, but we like to be helpful. The method is outlined in the box below in which we have distinguished two cases. #### Steps to obtain the P value from the CI for an estimate of effect ( Est ) ##### (a) P from CI for a difference

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Available abstract

We have shown in a previous Statistics Note 1 how we can calculate a confidence interval (CI) from a P value. Some published articles report confidence intervals, but do not give corresponding P values. Here we show how a confidence interval can be used to calculate a P value, should this be required. This might also be useful when the P value is given only imprecisely (eg, as P<0.05). Wherever they can be calculated, we are advocates of confidence intervals as much more useful than P values, but we like to be helpful. The method is outlined in the box below in which we have distinguished two cases. #### Steps to obtain the P value from the CI for an estimate of effect ( Est ) ##### (a) P from CI for a difference

Key concepts: Confidence interval, Value (mathematics), Statistics, CDF-based nonparametric confidence interval, Tolerance interval, Robust confidence intervals, Medicine, Interval (graph theory)

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